This invention relates to a noble metal-containing zeolite catalyst exhibiting enhanced high temperature resistance to agglomeration of the noble metal component(s) and to a process for preparing such a catalyst.
Noble metal-containing zeolites have been described for the catalysis of a variety of chemical conversion processes, e.g., hydroisomerization, disproportionation, hydrocracking, reforming, etc. When an aged zeolite catalyst is oxidatively regenerated to burn off accumulated carbonaceous deposits, or "coke", there is a tendency for the noble metal to migrate from the zeolite channels and to agglomerate in larger particles. This reduces the dispersion and the surface area of the noble metal available for contacting the reactants. To remedy this problem a process referred to as rejuvenation is employed in which a source of halogen such as chlorine is used to redisperse the noble metal. Frequently, chlorine rejuvenation makes matters worse, and the noble metal migrates from the zeolite to the binder material which is associated with the zeolite. When the noble metal hydrogenation component is far removed from the active acid sites of the zeolite framework, the catalyst loses selectivity. Fresh catalyst performance is rarely, if ever, obtained after current rejuvenation processes.
U.S. Pat. No. 5,041,401 to Schoennagel et al. discloses a zeolite catalyst composition resistant to agglomeration of its noble metal component by adding a non-framework multivalent metal oxide, e.g., alumina, in order to stabilize the noble metal. The non-framework component can be introduced by diffusion, impregnation, ion-exchange, and/or calcination in dry air (&lt;100 ppm water). In one embodiment, the non-framework alumina is derived from framework aluminum subjected to dry calcination conditions. The reference specifically teaches against calcining in air exceeding 100 ppm water (&lt;0.1 torr).